Hitherto, there have been mainly produced two types of golf balls. The one is a solid golf ball, such as a two-piece golf ball or a three-piece golf ball, which is composed of a core formed from integrally molded rubber material and a thermoplastic resin cover (e.g. ionomer resin cover) formed on the core. The other is a thread wound golf ball which is composed of a solid or liquid center, a thread rubber wound layer formed on the center and a cover of ionomer resin or balata (trans-polyisoprene) etc. having a thickness of 1 to 2 mm covering on the thread rubber wound layer. In the solid golf ball, a two-piece solid golf ball is composed of a core and a cover, and has been commercially available since 1982, because of easiness of production and the like. The two-piece solid golf ball has better durability and better flight performance because of larger initial velocity at the time of hitting and longer flight distance, when compared with the thread wound golf ball. Therefore, the two-piece solid golf ball is generally approved or employed by many golfers, mainly an average class of golfers.
On the other hand, the two-piece solid golf ball exhibits much harder shot feel at the time of hitting than the thread wound golf ball. Therefore, the two-piece solid golf ball is not approved nor employed by some users such as elderly persons or women who are non-muscular golfers. In order to improve shot feel, it has been attempted to soften the core by reducing a hardness of the core. The method improves the shot feel, but adversely sacrifices flight distance, because the rebound characteristics are degraded.
Recently, the two-piece solid golf ball has approved or employed by professional golfers or top amateur golfers other than the non-muscular golfers, because the two-piece solid golf ball has excellent flight performance and uniformity of quality between golf balls. The professional golfers or top amateur golfers require not only good shot feel but suitable controllability. The ionomer resin, which has been generally used for cover materials of two-piece solid golf balls, however shows high rigidity and high hardness, in order to impart sufficient flight performance and durability to the golf ball, and thus deteriorating controllability.
In order to solve the above problem, a golf ball, of which a cover is formed from a resin having low rigidity and low hardness, has been proposed. The reduction of rigidity and hardness of the cover improves controllability, but it is problem that the reduction degrades rebound characteristics of the cover, thus the resulting golf ball not having sufficient flight performance.
In order to further improve performances of the two-piece solid golf ball, there has been many proposals. In Japanese Patent Kokoku Publication Nos. 21426/1986, 44302/1993 and 98206/1994, and Japanese Patent Kokai Publication Nos. 182278/1990, 98949/1994, 154357/1994, 327792/1994, 194732/1995, 239067/1997, 104518/2001 and the like, two-piece solid golf balls are proposed, of which performances such as flight distance (rebound characteristics), durability, shot feel, controllability are improved by adjusting hardness and hardness distribution of core to proper ranges.
In Japanese Patent Kokoku Publication No. 21426/1986, a two-piece golf ball, of which hardness distribution is controlled such that a core has a hardness in JIS-C hardness at the surface layer of 72 to 78, a hardness in JIS-C hardness at the distance of 5 mm from the surface layer of 77 to 83, a hardness in JIS-C hardness at the distance of 10 mm from the surface layer of 72 to 80, a hardness in JIS-C hardness at the distance of 15 mm from the surface layer of 67 to 75 and a hardness in JIS-C hardness at the distance of more than 15 mm from the surface layer of not more than 75, is disclosed. However, in the golf ball, since hardness distribution near the surface of the core is not adjusted to a proper range, rebound characteristics are not sufficiently obtained.
In Japanese Patent Kokoku Publication No. 44302/1993, a two-piece golf ball having a deformation amount when applying from an initial load of 10 kgf to a final load of 130 kgf of 1.9 to 2.4 mm, of which hardness distribution is cotrolled such that a core has a hardness in JIS-C hardness at the distance of less than 10 mm from the center point of the core of 60 to 79 and a hardness in JIS-C hardness at the distance of more than 10 mm from the center point of the core of 80 to 90, is disclosed. However, in the golf ball, since the deformation amount of the golf ball is small, shot feel is poor.
In Japanese Patent Kokoku Publication No. 98206/1994, a two-piece golf ball, of which hardness distribution is cotrolled such that hardness difference in JIS-A hardness between the center point and a portion other than the center point of the core is within the range of −10 to 10%, is disclosed. However, in the golf ball, since the hardness difference between the center point and a portion other than the center point of the core is only considered, both good shot feel and excellent flight performance are not obtained.
In Japanese Patent Kokai Publication No. 182278/1990, a two-piece golf ball, of which core hardness is decreased substantially from the surface to the center point in order, and a relationship between the hardness and a distance from the surface of the core is adjusted to a specified range, is disclosed. However, in the golf ball, since the center hardness of the core is also high, shot feel is poor.
In Japanese Patent Kokai Publication No. 98949/1994, a two-piece golf ball, of which hardness distribution is controlled such that core hardness is increased at the center point, at the distance of 5 to 10 mm from the center point, at the distance of 15 mm from the center point and the surface in order, is disclosed. However, in the golf ball, since hardness distribution of the core is not adjusted to a proper range, shot feel is poor.
In Japanese Patent Kokai Publication No. 327792/1994, a two-piece golf ball, of which hardness distribution is controlled such that the core has a hardness in JIS-C hardness at the center point of 65 to 79, a hardness in JIS-C hardness at the distance of 5 mm from the center point of 70 to 80, a hardness in JIS-C hardness at the distance of 10 mm from the center point of 73 to 80, a hardness in JIS-C hardness at the distance of 15 mm from the center point of 75 to 82 and a hardness in JIS-C hardness at the surface of 70 to 85, and hardness difference between the two measuring points adjacent to each other in the core is within the range of −5 to 5, is disclosed. However, in the golf ball, since the hardness at the center point of the core is high, shot feel is poor.
In Japanese Patent Kokai Publication No. 194732/1995, a two-piece golf ball, of which a core has a center hardness in JIS-C hardness of 40 to 57, a surface hardness in JIS-C hardness of 70 to 95 and hardness difference between the surface hardness and center hardness of 20 to 40, is disclosed. However, in the golf ball, since the hardness near the center point of the core is significantly high, shot feel is poor.
In Japanese Patent Kokai Publication No. 239067/1997, a two-piece golf ball having hardness distribution such that a core has a surface hardness in JIS-C hardness of not more than 85, a center hardness in JIS-C hardness is lower than the surface hardness by 8 to 20 and a hardness at the distance of not more than 5 mm from the surface is lower than the surface hardness by not more than 8, is disclosed. However, in the golf ball, since the hardness distribution of the core is not adjusted to a proper range, shot feel is poor.
In Japanese Patent Kokai Publication No. 104518/2001, a two-piece golf ball comprising a core and a cover is disclosed. In the golf ball, assuming that a center hardness in JIS-C hardness of the core is A, a hardness in JIS-C hardness at the distance of not more than 5 mm from the surface of the core is B, a surface hardness in JIS-C hardness of the core is C, a hardness in JIS-C hardness of the cover is D, a distance between the center point and the point of not more than 5 mm from the surface of the core is K and a radius of the golf ball is L,                the value of A is within the range of not less than 58 to lower than 65, the value of B is within the range of 80 to 87, the value of C is higher than 78, the value of C is lower than that of B by 1 to 5, and        the values of A, B, D, K and L satisfy a relationship represented by the following formula:0.9≦{(D−B)/(L−K)}/{(B−A)/K}≦2.0However, in the golf ball, since the hardness distribution near the surface of the core is not adjusted to a proper range, rebound characteristics are not sufficiently obtained.        
It has been required to provide golf balls having good shot feel, of which the flight performance and durability are improved, by solving the problems. In addition, it is required to improve the performance of the golf ball still more.